The present invention relates generally to linear motion devices, and in particular to a linear brushless DC motor with an ironless armature assembly and substantially constant force throughout its stroke.
One of the simplest possible linear motion devices is a cylindrical linear voice coil actuator. However, these are limited stroke devices. If long strokes are required, a commutated linear voice coil actuator also known as a linear brushless DC motor can be used (FIG. 1A). An example of a linear brushless non-commutated device with flux-focused magnetic circuits can be found in the moving coil actuator disclosed in U.S. Pat. No. 5,345,206, assigned to BEI Electronics, Inc., the assignee of the subject application. U.S. Pat. No. 5,345,206 is hereby incorporated by reference into the subject application.
When linear motion devices are used in a servo system, the mass of a moving part (armature assembly) should be minimized. In addition, all the forces created by a motor, except for the force in the direction of motion, should be eliminated or minimized. And finally, to achieve a smooth operation, the force developed by a servomotor should remain constant throughout the stroke.
It is therefore an object of the present invention to provide a linear motion device that has a long stroke and moving parts of low mass.
It is another object of the present invention to provide a linear motion device in the form of a linear brushless DC motor in which the forces created by the motor, other than in the direction of motion, are minimized.
It is a further object of the present invention to provide a linear brushless DC motor having a smooth operation in which the force developed remains constant throughout the stroke.
It is a still further object of the present invention to provide a linear brushless DC motor having a smooth operation in which the force developed remains constant throughout the stroke by using a sinusoidal commutation of a three phase motor.
In accordance with the present invention there is provided a linear brushless DC motor with ironless armature assembly and a field assembly tailored to achieve a desired constant force versus stroke characteristic, for example, as set forth in FIG. 2.
The present invention comprises an armature assembly, and a field assembly that includes a plurality of permanent magnets each having a length, and a plurality of pole pieces each having a length. The ratio between the length of the plurality of permanent magnets and the length of the plurality of pole pieces is tailored to achieve a constant force versus stroke characteristic.
In accordance with the present invention, a brushless DC motor is provided comprising an armature assembly; and a field assembly positioned with respect to the armature assembly so that an air gap is formed between them. The field assembly includes a plurality of permanent magnets each having a length, and a plurality of pole pieces each having a length. The ratio between the length of the plurality of permanent magnets and the length of the plurality of pole pieces is selected to provide a sinusoidal distribution of a normal component of flux density in the air gap.
These and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention and the accompanying drawings.